The invention relates to an internal combustion piston engine and, more particularly, to a new two cycle engine construction which allows a greater proportion of work to be extracted from the hot gases and a method and assembly for converting a conventional two cycle piston engine to the new construction.
Conventional Otto and Diesel cycle internal combustion piston engines theoretically operate with equal compression and expansion ratios in the cylinder. The inefficiency of these conventional engines is principally due to the degree of heat in the gases rejected from the cylinder at the end of the expansion stroke. There is a theoretical cycle sometimes referred to as "partial charging" or the Atkinson cycle which allows a greater proportion of work to be extracted from the hot gases by causing the expansion ratio to be greater than the compression ratio. An engine operating in this type of cycle would have lower energy losses and hence improved thermal efficiency.
The conventional two cycle internal combustion engine is a reciprocating piston and cylinder arrangement wherein the engine chamber is defined between an end wall or head on the cylinder and the piston. The head is fitted with a plurality of exhaust valves for hot gas evacuation and a fuel injector for introducing fuel into the chamber. The opening and closing of the fuel injector and exhaust valves is controlled by cams on a rotary shaft. An air inducer, such as a Roots blower, serves to provide a supply of pressurized air in an intake manifold to be passed into the chamber. The air intake into the chamber is through inlet ports, laterally formed in the cylinder wall adjacent the bottom dead-center position of the piston in the chamber and controlled by the piston. The two cycle engine operation is such that, upon combustion, the piston commences its power stroke moving away from the cylinder head. The power stroke is completed upon opening of the exhaust valves which occurs before the piston reaches the inlet ports. As the piston continues movement away from the head, the inlet ports are uncovered to introduce pressurized air into the chamber. From beneath the inlet ports, the piston begins its stroke toward the cylinder head. As the piston covers the inlet ports, the exhaust valves are closed and the piston starts its compression stroke, leading to combustion. The use of only sidewall parts for an intake requires that the compression stroke of the piston be equal or very nearly equal to the expansion or power stroke of the piston.
Heretofore, little consideration has been paid to two cycle engines for Atkinson cycle operation. It has not been practicable to utilize the Atkinson cycle in a conventional two cycle engine construction, since a considerable power loss ensues. The expansion ratio can be made greater than the compression ratio by holding open the exhaust valves for some time after the inlet ports have been covered by the piston; however there is substantial loss of pressure in the intake air which remains in the chamber after the exhaust valves are finally closed.
The present invention concerns a two cycle engine construction which is capable of practical performance with an expansion ratio greater than the compression ratio. By operating the two cycle engine of the present invention on a fundamentally more efficient thermal cycle, specific fuel consumption is improved and the engine runs cooler and is less polluting. The present invention also concerns a method and assembly for modifying standard two cycle engine constructions such that the resultant two cycle engine is capable of practical performance with an expansion ratio greater than the compression ratio.